Every way a modded server dies

24 failure modes, each with what it means and the fix. Same root causes and remedies the tool prints — find yours below.

Attempted to load class for invalid dist DEDICATED_SERVER — Client-only code reached the dedicated server
mod has been loaded on the wrong server side — A client-only mod is installed on the server
Missing or unsupported mandatory dependencies Minecraft server — A mod is missing a dependency
Found duplicate mods Forge server — The same mod is installed twice
Mixin apply failed InvalidInjectionException — A mixin failed to apply
Failed to check session lock minecraft server — The world folder is already in use
UnsupportedClassVersionError class file version Minecraft — Wrong Java version
java.lang.OutOfMemoryError Java heap space Minecraft server — Out of memory
java.net.BindException Address already in use Minecraft — The port is already taken
You need to agree to the EULA in order to run the server — EULA not accepted
java.lang.ExceptionInInitializerError Minecraft mod — A class failed during static initialisation
Unknown registry entries Minecraft world — World contains entries for mods that are gone
mod is for mod loader which is not compatible — Mod built for a different loader version
A single server tick took 60.00 seconds ThreadStuckError — The server hung and the watchdog killed it
java.lang.NoSuchMethodError Minecraft mod — Mods compiled against mismatched versions
ConfigLoadingException Forge server — A config file is invalid
Mod rejections connection closed Minecraft — Players are being kicked for a mod mismatch
Couldn't parse data file recipe Minecraft — A datapack or recipe failed to load
NoClassDefFoundError Minecraft mod stale jar — A mod is missing one of its own classes (stale or partial jar)
ResourceLocationException Non [a-z0-9_.-] character in path — An invalid resource ID was loaded from config or a datapack
Ticking entity crash Minecraft server mod — A mod crashed while ticking an entity or block
java.lang.Exception Mod Loading has failed — Mod loading failed — the real cause is further down this report
IllegalStateException Failed to initialize server Minecraft — Server failed to initialise — look for the Caused by
is not a valid mod file Forge — A jar in mods/ isn't a valid mod